1. Field of the Invention
This invention relates generally to systems for locating and tracing buried objects and more particularly to a system for inducing alternating current in a buried conductor to facilitate the detection and tracing thereof by means of an automated signal detection system.
2. Description of the Related Art
There are many situations where is it desirable to locate buried utilities such as pipes and cables. For example, before starting any new construction that involves excavation, worker safety and project economic concerns require the location and identification of existing underground utilities such as underground power lines, gas lines, phone lines, fiber optic cable conduits, cable television (CATV) cables, sprinkler control wiring, water pipes, sewer pipes, etc., collectively and individually herein denominated “buried objects.” As used herein, the term “buried objects” includes objects located inside walls, between floors in multi-story buildings or cast into concrete slabs, for example, as well as objects disposed below the surface of the ground. If excavation equipment such as a backhoe hits a high voltage line or a gas line, serious injury and property damage may result. Unintended severing of water mains and sewer lines generally leads to messy and expensive cleanup efforts. The unintended destruction of power and data cables may seriously disrupt the comfort and convenience of residents and bring huge financial costs to business.
Accordingly, the art is replete with proposed solutions to the buried object locating problem. For example, it is known to locate buried ferromagnetic objects either by detecting a localized change in free-space permeability with an inductive circuit element or by using a magnetic sensor to detect the fixed magnetic field (internal magnetic moment) emanating from the object. The presence of such ferromagnetic objects also distorts the Earth's magnetic field in a manner that is known to have utility for locating. Some buried cables, such as power lines and some communication lines, for example, are already energized and therefore characterized by the emission of an electromagnetic signal that includes a non-zero frequency magnetic field that may be detected by a magnetic sensor. As another example, an external transmitter, beacon or duct probe (a “sonde”) is an external electromagnetic signal source having well-known utility for marking the location of any non-conductive buried object into which it may be physically introduced. A sonde typically includes a coil of wire wrapped around a ferromagnetic core that is packaged for insertion into a buried nonconductive conduit, such as a plastic utility runway or a concrete water pipe. Still other buried objects, such as conductive lines and pipes, may be located by first applying an external electromagnetic signal to the object to induce an alternating current therein, thereby energizing the object with a nonzero frequency magnetic field that may be detected by a magnetic sensor. For example, an external electrical signal source (transmitter) having a frequency in the range of approximately 4 Hz to 500 kHz has a well-known utility for energizing conductive objects by direct electrical coupling to permit their location. These examples of active and passive location of buried long conductors are also commonly denominated “line tracing.”
The above commonly-assigned patent applications propose several improvements to the magnetic field measurement and line locating art, including the use of simultaneous measurement of magnetic field vectors in a plurality of independent frequency regions and the introduction of multiple 3D sensor arrays for measuring magnetic field vectors and the introduction of an improved Graphical User Interface (GUI) for line tracing.
Employing a directly-coupled external transmitter to induce an alternating current in a buried conductive objects is useful, provided that the buried line is accessible for the conductive attachment of the transmitter output signal. When there is no conductive access to the buried conductor, such a transmitter may alternatively be used to indirectly induce an alternating current in the buried line, but this approach has several well-known disadvantages. For example, when the transmitter is placed at a first location above the buried line and operated to radiate an electromagnetic field in the 4 Hz to 500 kHz region, an alternating current is induced in the line such that a locator array tuned to the same frequency may be deployed at a second location removed from the transmitter to detect the buried line. Any electromagnetic field energy that propagates above ground to the locator array directly from the transmitter tends to interfere with the accuracy of the desired locator measurement. However, the above-ground transmission of energy is significantly less efficient than the underground transmission, which arises from an electrical current flowing in a continuous conductor. This indirect-coupling active location technique is therefore useful over a range of transmitter and locator separations.
The minimum requisite separation between locator and transmitter may be reduced by providing the transmitter with a directional radiation pattern and directing it orthogonally to the locator direction. The maximum feasible separation between locator and transmitter is generally proportional to the transmitter output energy that may be successfully coupled to the underground line. This in turn depends on the efficiency with which the transmitter converts available battery power to induced magnetic flux. If the metallic line is buried deeply, a transmitter positioned on the surface must produce a high magnetic flux to penetrate to the line depth. In addition to a high transmitter flux, the magnetic flux pattern should also be widely dispersed in the downward direction to reach a deeply buried line. Thus, there is a recognized need in the art for a strong and effective induced signal, which must be provided by battery-powered transmitter apparatus that is easily portable.
U.S. Pat. No. 5,055,793 issued to Mulcahy discloses a battery-powered transmitter circuit supported from a portable housing, a ferrite core antenna incorporated within the transmitter circuit, and a separate tank circuit that includes a loop antenna and a capacitor. The tank circuit is tuned to a predetermined operating frequency and nonconductively coupled to the ferrite core antenna, which transfers magnetic energy from a signal generator to the loop antenna. The magnetic field induced from the loop antenna inductively coupled to the buried line to create an alternating current therein. Mulcahy neither considers nor suggests solutions to the problem of tuning drift, which arises from temperature changes and aging in tank circuits and can reduce transmitter efficiency and magnetic flux output. Moreover, Mulcahy's apparatus must be returned to operate at an alternate frequency, which is often necessary in modern systems intended for use where the land that is to be excavated is traversed or crisscrossed by several different utilities such as an electrical power cable, a water line, a gas line, a sewer pipe and a communications line. It is highly desirable to be able to determine their paths and their depths all at the same time by using, for example, several different signals at different frequencies for application to the same underground object or even to different underground objects.
Accordingly, there is still a clearly-felt need in the art for an improved compact self-tuned electrical resonator suitable for generating a high magnetic flux from a battery-powered generator. These unresolved problems and deficiencies are clearly felt in the art and are solved by this invention in the manner described below.